1. Field of the Invention
The present invention relates to a power converter, which is connected between a power source and a load, converts a direct current to an alternate current by a plurality of switching elements, performs an inverse conversion thereof, and thereby controls power to the load; and particularly, a technology for reducing an electromagnetic wave interference generated in operation of the switching elements.
2. Description of the Related Art
In a power converter, based upon a high voltage resistance and high speed of a switching element such as an IGBT (Insulated Gate Bipolar Transistor), a switching voltage increases, a high frequency of a frequency proceeds, and an efficiency improvement and a cost reduction proceed. Generally, it is known that a switching of the power converter derives a current leaking to the earth called a common mode current through a parasitic capacitance among a load and power source, which are connected to the power converter, an electric wire for connecting those, and the earth. In addition, also in an electronic component of inside of the power converter, a switching resonates a parasitic capacitance between component housings or between a parasitic capacitance of the component housings and a power converter housing and a parasitic inductance of wiring due to a fluctuation of an electric potential in an inner conductor by the switching; it becomes a leak noise for an outside; and thereby causes an electromagnetic interference of external appliances.
Also in a case of an electric vehicle for a railway a switching of an inverter (power converter) causes an induced interference for a train wireless apparatus and signaling apparatuses, and a radio audible interference. In the electric vehicle for the railway the switching of switching equipment for driving a motor leaks a high-frequency current through a motor wire and a motor parasitic capacitance.
Conventionally, as one example, a means for solving this problem inserts a choke core filter, which uses a magnetic core, in a motor wire, makes an impedance of the motor wire become higher as a frequency becomes higher, and thereby reduces a leak high frequency.
In addition, as described in Japanese Patent Laid-Open Publication Hei 10-313501, even when grounding a housing of a power converter to a current collector of a negative pole, grounding the housing to a vehicle body, and concentrating a high-frequency current in vicinity of the housing of the power converter, the induced interference is reduced by connecting a resistor between the vehicle body and an earth brush and thus recovering the high-frequency current with passing it through the vehicle body. This uses a fact that a wiring inductance of the vehicle body is small, comparing a wiring inductance of a main circuit electric wire with that of the vehicle body. It is possible to suppress the induced interference to a wireless apparatus and the signaling apparatuses by positively recovering the high-frequency current and suppressing a potential fluctuation of a switching circuit of the power converter, the housing of the power converter, and the vehicle body, which fluctuation is due to the high-frequency current passing the wiring inductance of the main circuit electric wire.
Furthermore, as described in Japanese Patent Laid-Open Publication 2002-136153, taking notice of the parasitic capacitance of a module within switching equipment connected to cooling equipment and making a bypass circuit of the high-frequency current, there is a method of blocking the high-frequency current within a housing of the power converter. In FIG. 2 is enlargedly shown a housing portion. Cooling equipment thermally contacts switching equipment to radiate heat of a switching element within a switch module. Therefore, the cooling equipment results in being connected to a motor wire through the parasitic capacitance of the module. Then some percent of high-frequency currents passing the motor wire can be recovered as a high-frequency current by connecting the cooling equipment and a negative pole terminal of the power converter through a conducting wire and a resistor. Thus it is possible to suppress the induced interference.
Even applying the conventional technologies described above, it is not sufficient to suppress the induced interference of signaling apparatuses, which use a frequency of not less than 1 MHz, and the radio audible interference within a vehicle.
When using a choke coil, there are frequency characteristics in a magnetic core used for the choke coil and the characteristics drastically degenerates in the frequency of not less than 1 MHz. In addition, because a material amount of the magnetic core for making a necessary inductance increases, there is a problem that a cost and a space increase.
Because when connecting a resistor between a vehicle body and an earth brush, a high-frequency current flows in the vehicle body, it results in giving interference to a radio audibility within the vehicle.
Next, taking notice of the parasitic capacitance, a method, which connects cooling equipment and an inverter negative pole terminal with using a conducting wire, may delay a voltage change speed of a switching to some extent and lessen the high-frequency current. However, if because an inner impedance is small within a voltage source where a noise source is a pulse form, a bypass circuit of a low impedance is provided in parallel with a motor wire and a parasitic capacitance circuit, it does not almost influence on the high-frequency current flowing in the motor wire and the parasitic capacitance circuit although the high-frequency current flows in the bypass circuit depending on the impedance. The current rather results in flowing in the bypass circuit of the low impedance, and it results in forming a new high-frequency current circuit. That is, the induced interference and the radio audible interference cannot be suppressed.
Subsequently, is strongly requested a power converter, which can suppress the induced interference and the radio audible interference by not leaking a high frequency current of the power converter outside, based upon a switching of switching equipment, and being able to recover the high frequency current in a short path through a low impedance path